What is an endothermic reaction ?
An endothermic reaction is a type of chemical reaction in which energy is consumed in the form of heat, so that the product obtained has an energy greater than the initial reagents.
The term endothermic has Greek roots: “endo”, which means inside, and “thermos”, hot, which would mean absorbing heat.
To understand how an endothermic reaction works, it is necessary to understand the concept of enthalpy, which is the amount of energy that a given system can absorb or release into the environment. Then, in an endothermic reaction, the enthalpy variation is greater than zero (?H> 0).
As the resulting products are more energetic than the initial reagents, energy absorption from the environment is required to meet that need. This process generates a break in the active bonds, and that is when the endothermic reaction occurs.
The energy absorbed in the form of heat has to be taken from somewhere, in this case, from the immediate environment or environment, so in many endothermic reactions the process implies a temperature drop.
18 Examples of endothermic reactions
- To make a bread requires a source of heat, which will be absorbed by the dough so that the necessary chemical reactions occur that will give as a final product a baked bread.
- Ozone production : oxygen molecules absorb energy from ultraviolet radiation and to become ozone (0 3 ).
- Obtaining iron sulfide : to obtain this component it is necessary to combine iron with hydrogen sulfide, so it is necessary to apply heat to stimulate the endothermic reaction.
- Hydrolysis : is the process in which the hydrogen and oxygen molecules that make up the water, are separated by the action of electrical energy.
- Cooling a bottle of warm water while it is in full sun, if it is covered with a damp cloth.
- The thaw of the pipes that froze is very problematic. In some very common cases, a real puzzle, when they are made of PVC plastic material , PE or PER , or also when they are coated with a trim or recessed. Similarly, when it comes to valves, faucets, valves that have fragile joints. In fact, the propagation of the heat necessary for thawing can also be hampered by a very limited thermal conductivity . The effect of enclosure is also there to oppose the thaw.
- Photosynthesis . The equation : 6CO 2 (g) + 6H 2 O (1) + energy =
C 6 H 12 O 6 (s) + 6O 2 . For the reaction to take place, energy must always be supplied
- (Certainly, it releases energy when new bonds are formed in the sugar molecule, but the energy released is less than the energy that must be added to break the bonds in the carbon dioxide and in the water.)
The reaction is called endotherm (of gr. Endo = in).
- The total (chemical) energy of the products is higher than that of the reactants.
- Ammonium nitrate is dissolved in water . The reaction equation:NH4NO3(s) + water =NH4+(aq) + NO3–(aq) .This is an example of endothermic reaction because the temperature drops because heat energy is taken in by the reaction mixture .
- Fusion of snow on a warm windshield (especially for heavy snow-wet) . In these cases the contact surface is very much increased. The flakes, which then react with a “sponge effect”, keep a liquid phase in the flake. On the other hand, the crystal surface is much smaller. This causes a large cooling flow through the windshield, powered by the endothermic fusion. As proof in this case, the steam inside is very difficult to eliminate by hot ventilation. Canadians do not have the right to ride with a layer of old snow on their car. The regulation requires them preventive cleaning, before starting, under penalty of contravention.
- Sublimation of carbon dioxide (dry ice).Carbon dioxide as a solid is called dry ice it sublimates and becomes gas at -78.5 °C (-109.3°F).
- Fusion of ice and snow on roads in winter, especially when it is caused by salting . Salting significantly reduces the melting temperature of the ice ( eutectic mixture ), but also the increase of the exchange surfaces. For this reason, these salting operations should be avoided in the evening, night cooling could amplify the freeze. The leached road and its salt removed, could then freeze, for example by capturing water vapor, reforming ice (called black).
- Dissolving ammonium chloride (which is a salt ) in water . Here is the equation NH4Cl(s) + H2O(l) = NH4+(aq) + Cl-(aq) + H2O(l)
- Electrolytic decomposition of sodium chloride into hydrogen chloride and sodium hydroxide
- Mixing water with potassium chloride . During this reaction no heat is produce because the potassium chloride absorbs the surrounding heat .
- Dissolving solid Barium hydroxide and solid ammonium chloride .If we mix solid barium hydroxide with solid ammonium chloride an endothermic reaction (temperature reduces to -20 °C. ) takes place it produces a liquid, with the evolution of ammonia. The equation :Ba(OH)2.8H2O(s) + 2NH4Cl(s) = 2NH3(g) + 10H2O(l) + BaCl2(s)
Ba(OH)2.8H2O(s) + 2NH4Cl(s) = 2NH3(g) + 8H2O(l) + BaCl2.2H2O(s
- Cooking an egg
- Glycerol chemical reaction with fatty acids to make the lipid triglyceride in coconut oil .
- Cracking of alkanes . Cracking can be defined as the chemical process by which a chemical compound (usually organic) that breaks down or split into simpler compounds.
Here is an example with the equation: hexane cracked to form butane and ethene:
hexane ? butane + ethene
C6H14 ? C4H10 + C2H4